1. Field of the Invention
This invention relates to an impact printer, and more particularly, to an impact printer provided with a plurality of electrostatic capacitance sensors within a wire dot print head (hereinafter simply "print head"), to monitor the operations of printing wires.
2. Description of Related Art
An impact printer has a print head mounted on a carriage. Within the print head are provided a plurality of electrostatic capacitance sensors for monitoring the operations of printing wires fixed to the tips of armatures that oppose cores of respective electromagnets. The printing wires are operative to print dots on a printing medium by driving coils of the respective electromagnets, the coils being wound around the respective cores. The output of an electrostatic capacitance sensor are used to control the length of time that a coil is driven, thereby to improve the quality of printing. Such impact printers also use the electrostatic capacitance sensors for detecting respective gaps between the printing medium and the printing wires. The detection of gaps serves, among other things, to distinguish the type of printing medium. For example, detection of the gap between the printing wires and the printing medium may be used to distinguish the printing medium as between single sheets of paper and four-sheet-duplication paper. Such detection of gaps and determinations of printing medium are used to adjust the gaps to optimum values for improved printing quality.
During printing, a gap between an ink ribbon and each of the printing wires will vary depending on the type of printing medium, for example, whether the printing medium is a single sheet of paper or is four-sheet-duplication paper. Thus, the gap is wider when the printing medium is a single sheet of paper than when the printing medium is four-sheet-duplication paper. As a result, the printing wires will operate differently in each case. That is, the time duration of a stroke of a printing wire (hereinafter "impact time"), upon which the printing wire strikes the printing medium, is longer when the printing medium is a single sheet of paper than when the printing medium is four-sheet-duplication paper. Using the relationship of impact time to paper type, the impact printer can distinguish the type of printing medium, as well as detect the thickness of the printing medium. The printer adjusts the gap to have an optimum value that optimizes the control of the armatures of the printer for the particular type of printing medium. This optimum value is established in relation to a standard gap between each printing wire and the platen of the printer at the time that the printer is assembled.
The information which indicates the standard gap (hereinafter called "standard information value") is measured under predetermined conditions and stored in a non-volatile memory of the printer, so that the standard information value can be preserved as an initially stored value even if a power source for the printer is cut off. The standard information value of conventional impact printers cannot be adjusted or renewed during repeated use of the printer when once set to an optimum value.
In such a known impact printer, however, it is difficult to maintain the optimum gap after repeated use of the printer. In such a printer, alternating deactivations and activations of a coil cause the strokes of the printing wire toward and away from the platen. Upon each stroke the attracting end of the core contacts, and therefore slowly abrades, a side of the armature. Abrasion of the armature results in a lengthening of the stroke of the printing wire. Also, the gap between the printing wire and the platen is lengthened. Therefore, even though the gap initially is set to an optimum value, at a time that the impact printer is built and adjusted, repeated printing cause the gap to be increased and therefore to become greater than the optimum value.
Moreover, the optimum gap may be changed by wear of the tip of the printing wire and of the carriage, and by external impacts exerted upon the printer. Also, by such wear and impacts, the impact time of each printing wire, as detected on the basis of the output of the corresponding electrostatic capacitance sensor, changes, thereby reducing the accuracy with which gaps are detected and with which the types of printing medium are distinguished.